Frequency modulated-pulse type radio locator



Dec. 30, 1947. KEISTER 2,433,669

i REQUENCY MODULATED-PULSE TYPE RADIO LOCATOR Filed Aug. '21, 194-4 I 1. E AMPLIFIER 3a PULS/NG cmcu/r 43. I L W -4 I 5+ I 66 I 1:

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PULSE SWEEP I M? AMPLIFIER DETECTOR MPUFIER mspmv CIRCUIT Inve ntor: James E. Keister,

His Attorn ey.

Patented Dec. 30, 1947 FREQUENCY MODULATED-PULSE TYPE RADIO LOCATOR James E. Keister, Scotia, N. Y., assignor to General Electric Company, a corporation of New York Application August 21, 1944, Serial No. 550,366

2 Claims. 1

The present invention relates to locating equipment comprising means for transmitting pulses of radio waves and for receiving echoes of the transmitted pulses from remote objects.

An object of my invention is to provide an improved arrangement for utilizing the transmitter as part of the receiver.

The novel features which I believe to be char acteristic of my invention are setforth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 shows radio locating equipment having a transmitter which oscillates at a difierent frequency during the intervals between the transmitted pulses, the different frequency being applied to a mixer together with the echoes to produce an intermediate frequency, and Fig. 2 shows radio locating equipment having a transmitter which oscillates at a different frequency between the transmitted pulses, the transmitter being arranged as an oscillating mixer.

Referring to the drawing there is shown radio locating equipment having a microwave (e. g., 3,000 megacycle) transmitter I and a mixer 2 respectively connected by concentric transmission lines 3 and 4 through a junction 5 to a directional antenna 6. The transmitter comprises a reentrant cavity oscillator having an ultra high frequency triode l which is shown in elevation with the cathode 8, grid 9, and anode l0 diagrammatically indicated in dotted lines. The cathode is directly connected to an outer metal cylinder II, the grid 9 is connected by a metal flange l2 to a concentric inner metal cylinder I3, and the anodelfl is connected by a sliding contact [4 to an axially movable metal rod l5 carrying a tuning disk I6 having a flange I'l capacity-coupled to the outer metal cylinder I I. The tuning of the oscillator is varied by moving the rod l5 in and out.

The mixer 2 has an annular cavity l8 and an associated ultra high frequency triode 19 shown in elevation with the cathode 20, grid 2|, and anode 22 diagrammatically indicated in dotted lines. The cathode is capacity-coupled as indicated at 23 to a metal shell 24 telescoped within an upstanding sleeve 25 at the center of the cavity I8, the grid 2| is connected to a terminal flange 26 engaging contact fingers 21 on a ground plate 28 which serves as a bottom wall Of the cavity, and the anode 22 is connected to a terminal 29. The transmission line 4 is coupled to the cavity I8 through a loop 30 connected to the ground plate 28 and the center conductor of the transmission line.

The transmitter is excited by a pulsing circuit 3| connected to the rod 15 and arranged to apply high voltage pulses to the anode ID at the desired repetition rate. The rod 15 is also connected through a reactor 32 to a lower voltage supply which causes oscillation of the transmitter at a difierent frequency during the intervals between the high voltage pulses. The difference in the transmitter frequency for the two modes of excitation is substantiall constant for a particular set of operating conditions and is substantially independent of temperature variation. It has been found that changing transmitter tubes changes the frequencies so that retuning of the transmitter is required but the frequency difference remains constant. The transmitter oscillations (the high intensity oscillations during the periodic pulsing and the continuous lower intensity oscillations during the intervals between pulsing), are picked up by a loop 33 in the anode grid cavity and transmitted to the antenna 6 along the transmission line 3. The oscillations, upon reaching the junction of the transmission lines 3 and 4, divide, part flowing to the antenna 6 and the remainder flowing along the transmission line 4 to the mixer 2. During pulsing of the transmitter for transmitting it is desirable that the part of the pulse flowing along the transmission line 4 be kept to a minimum so that the greater part of the pulse will be transmitted to the antenna. This is eiiected by connecting the cathode 20 of the mixer to the pulsing circuit 3i so as to apply a coincident positive pulse sufiicient to bias the triode I9 off during pulsing of the transmitter. When the triode I9 is off, the flow of energy through the triode is prevented and the transmission line 4 is terminated by the coupling loop 30. By means of a line stretcher 34 by which the length of the line may be tuned to desired lengths, the transmission line 4 is tuned under this condition to an odd multiple of quarter wavelengths so that the short circuit of the coupling loop 30 causes a high impedance to appear at the junction 5 due to the impedance inversion characteristics of quarter wavelength transmission lines.

During the intervals between the transmitted pulses the continuous oscillations of the transmitter, due to the low voltage excitation through the reactor 32, are conducted through the transmission line 4 to the mixer. Since the mixer 3 triode i9 is conducting during these intervals, the termination of the transmission line 4 consists of the impedance of the loop 39 and the impedance coupled to the loop through the triode Is. This changes the tuning of the transmission line 4 so that it no longer presents a high impedance to the flow of energy from the junction 5. Echoes or reflections from remote objects arriving during the intervals between the transmitted pulses are picked up by the antenna 6 and conducted through the transmission line 4 to the mixer. The difference between the echo frequency and the trans mitter frequency produces a beat frequency in the mixer which is fed through and I. F. amplifier 35, a detector 36, and a pulse amplifier 31 to display equipment 38 associated with a sweep circuit 39 triggered by the pulsing circuit. By means of a line stretcher 40 the transmission line 3 is tuned so that during the intervals between transmitted pulses when the transmitter is excited at a low voltage, a high impedance appears at the junction to the flow of energy along the transmission line 3 from the antenna. The greater part of the echo strength is accordingly fed to the mixer and is used for receiving purposes.

In Fig. 2 is shown equipment in which the pulse transmitter is used as an oscillating mixer during the intervals between the transmitted pulses. This arrangement simplifies the equipment. Corresponding parts are indicated by the same reference numerals. As in the previously described construction, the pulsing circuit 3| is connected to the rod [5 to apply high voltage pulses to the anode I0 at the desired repetition rate, causing high intensity oscillation of the transmitter which is fed through the coupling loop 33 to a transmission line 4| connected to a directional antenna 42. During the intervals between the transmitted pulses the transmitter is excited at a lower voltage through the reactor 32, causing lower intensity continuous oscillation of the transmitter at a different frequency. As in the previously described construction, this different frequency is constant and beats with the echoes picked up by the antenna and fed to the transmitter through the coupling loop 33 to provide an intermediate frequency which is taken from the anode circuit of the transmitter by a transformer 43. An intermediate frequency by-pass condenser 44 is arranged between the transformer and the pulsing circuit. The transformer output is fed through the I. F. amplifier 35, the detector 36, and the pulse amplifier 31 to the display 38 associated with the sweep circuit 39 keyed by the pulsing circuit 3|.

In both equipments the transmitter has two frequencies of oscillation, one of which is utilized for the transmitted pulses and which therefore corresponds to the echo frequency, and the other of which is present during the intervals between the transmitted pulses and is used to beat with the echoes to provide an intermediate frequency for receiving purposes. By this arrangement the 4 necessary components of the equipment are reduced in number.

While I have shown particular embodiments of my invention, it will be understood that many modifications may be made without departing from the spirit thereof, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention,

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In radio locating equipment in which impulses are transmitted and in'which echo impulses are received after reflection from an object in space, an oscillator, means for alternately exciting the oscillator at a low intensity level at which the oscillator has one frequency of oscillations and pulsing the oscillator at a high intensity level at which the oscillator has a different frequency of oscillation, radiating means coupled to the oscillator for transmitting high intensity pulses only during periods of said high intensity level and for receiving echoes of the pulses during periods of said low intensity level, and means for beating said echoes with the oscillations of said first frequency to produce an intermediate frequency, a detector for said intermediate frequency, and means for interrupting the flow of energy to said detector during said periods of high intensity.

2. In radio locating equipment in which impulses are transmitted and in which echo impulses are received after reflection from an object in space, an oscillator, means for alternately exciting the oscillator at a low voltage at which the oscillator has one frequency of oscillation and pulsing the oscillator at a higher voltage at which the oscilaltor has a different frequency of oscillation, radiating means coupled to the oscillator for transmitting high intensity pulses only during periods of said higher voltage and for receiving echoes of the pulses during periods of said low voltage, and means for beating said echoes with the oscillations of said first frequency to produce an intermediate frequency, an amplifier and detector for said intermediate frequency, and means for interrupting the flow of energy to said detector during said periods of higher voltage.

JAMES E. KEISTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,280,226 Firestone Apr. 21, 1942 FOREIGN PATENTS Number Country Date 111,594 Australia Mar. 19, 19.3.9 

